A First Top Partner Hunter's Guide

TL;DR

This work develops a minimal, symmetry-driven effective field theory for the lightest top partners in a composite Higgs framework with a pNGB Higgs from the SO(5)/SO(4) coset. By enforcing CCWZ structure and partial compositeness, it builds four concrete models (M4_5, M4_14, M1_5, M1_14) describing the spectrum and couplings with a small parametric set (M_Ψ, f, ξ, y, c1, c2), and details how the top Yukawa and Higgs potential arise from symmetry-breaking spurions. The paper then analyzes top-partner phenomenology, quantifies both pair and single production channels, and derives LHC bounds by recasting CMS 4th-generation quark searches, highlighting how current data probe naturalness in these models and identifying experimental strategies—such as forward-jet tagging and targeted single-production searches—that could further enhance sensitivity. Overall, it provides a practical, semi-analytic framework to interpret collider bounds in terms of naturalness and the light top-partner spectrum. The results emphasize that LHC data already probe the natural region, while also outlining avenues for improved discrimination of top-partner scenarios in future runs.

Abstract

We provide a systematic effective lagrangian description of the phenomenology of the lightest top-partners in composite Higgs models. Our construction is based on symmetry, on selection rules and on plausible dynamical assumptions. The structure of the resulting simplified models depends on the quantum numbers of the lightest top partner and of the operators involved in the generation of the top Yukawa. In all cases the phenomenology is conveniently described by a small number of parameters, and the results of experimental searches are readily interpreted as a test of naturalness. We recast presently available experimental bounds on heavy fermions into bounds on top partners: LHC has already stepped well inside the natural region of parameter space.

Figures (4)

• Figure 1: Schematic picture of the spectrum.
• Figure 2: The typical spectrum of the top partners.
• Figure 3: The single-production diagrams.
• Figure 4: In red dashed: the cross sections of pair production. In green and blue the single production of the $\widetilde{T}$(in association with a $b$) and of the $X_{5/3}$(in association with a $t$), respectively in model M1$_{5}$ and M4$_{5}$. The point chosen in the parameter space is $\xi=0.2$, $c_1=1$ and $y=1$. The value of $c_2$ is fixed, at each value of $M_\Psi$, in order to reproduce the top quark mass.